10 things you can do to make your app secure: #3 Validate Input

This is part #3 of a series of posts on the OWASP Top 10 Proactive Development Controls.

Your first line of defence against attacks should always be to check all data from untrusted sources. Input validation is fundamental to application security, and a basic part of good defensive programming.

This is simple, and obvious – and often done wrong.

Don't Rely on Client-Side Validation

One common mistake is relying on client-side validation to catch problems. Client-side validation is useful in providing immediate feedback at the UI. But it won’t protect your system, especially in web apps. If an attacker can find a way to insert themselves in between the browser and your app, which can be done using a proxy, they can play with any of the data, including header fields and other hidden data, after local client-side editing has already been done. Data from a client, especially a client outside of your network, should never be trusted.

Another common mistake is relying on negative checking or “black list” validation to try to catch known bad data. This is a weak technique: you can’t be sure that you will catch all of the bad things or combinations, and negative checks can be subverted through (double) encoding and other evasion tricks.

White List Validation

OWASP recommends that you always use positive restrictions – white list validation – where you define the acceptable type and size, and all allowable values using regular expressions (regex). An example from the Proactive Controls: the regex for a password field could be:

^(?=.*[a-z])(?=.*[A-Z]) (?=.*\d) (?=.*[@#$%]).{10,64}$

This regular expression ensures that a password is 10 to 64 characters in length and includes a uppercase letter, a lowercase letter, a number and a special character (one or more uses of @, #, $, or %).

White list validation is trivial for enumerated data: days of the week, months of the year … anything that you can fit into a defined list. For other common data types (such as dates and time zones, numeric types, currencies, email addresses, IP addresses, URLs and domain names, and credit card numbers), you could use the Apache Commons Validator library.

Validation of free-format text fields (names, comments) is more difficult, and especially text fields that can – or need to – contain special characters, especially markup:

If your application handles markup -- untrusted input that is supposed to contain HTML -- it can be very difficult to validate. Encoding is also difficult, since it would break all the tags that are supposed to be in the input. Therefore, you need a library that can parse and clean HTML formatted text such as the OWASP Java HTML Sanitizer. A regular expression is not the right tool to parse and sanitize untrusted HTML.

You can test how solid your input validation is with different tools, such as fuzzers or static analysis taint checking (using tools that run through execution paths in the code and identify when you are referencing data that has not been validated) and through pen testing and manual exploratory testing.

Next: Access Control. How to do it, and how not to do it.